Seamless copper alloy tube for heat exchanger being excellent in 0.2% proof stress and fatigue strength

ABSTRACT

A seamless pipe made of copper alloy consisting of, by weight % , a total amount of 0.02 to 0.2% of Co, 0.01 to 0.05% of P, 1 to 20 ppm of C if needed, and remainder Cu, and unavoidable impurities and, as said impurities, the total oxygen content is regulated to 50 ppm or less, and useful for a heat transfer pipe of a heat exchanger and especially, when HFC-type fluorocarbon is used as a heating medium.

This application is a 371 of PCT/JP99/03118, filed Jun. 11, 1999.

BACKGROUND OF THE INVENTION Field of the Invention:

The present invention relates to a seamless copper alloy pipe havinghigh 0.2% proof strength and fatigue strength which is mainly used for aheat transfer pipe of a heat exchanger and especially relates to saidpipe which can be used as a heat transfer pipe when HFC-typefluorocarbon is used as a heating medium.

Discussion of the Background:

In general, a seamless pipe of phosphorus deoxidized copper has beenused as a heat transfer pipe of a heat exchanger. In order to assemblethe pipe of phosphorus deoxidized copper above to a heat transfer pipeof a heat exchanger, at first, said pipe should be cut to predeterminedlength and be formed to U character form by bending. After that, this Ucharacter formed pipe is passed into through holes of aluminum oraluminum alloy fins which are arranged in parallel and these fins arefixed on said pipe in parallel by extending a inside diameter of saidpipe to pass through a plug or to load liquid pressure.

Moreover, the end of the U character formed pipe above is extended byflare forming and re-flare forming which is extending again the alreadyflare formed end of the pipe and these extended ends are combined withother U character formed pipes by inserting one end of a not-extended Ucharacter formed pipe into the other extended end of pipe and solderingeach other using a phosphorus copper solder.

After extending the end of pipe, when the end of U character formed pipeof usual phosphorus deoxidized copper is heated in soldering, crystalgrowth of copper alloy at heated area arises and, as a result, thestrength of said heat influenced area which adjoins at soldered part mayfalls remarkably. As a seamless copper alloy pipe for a heat exchangerto prevent crystal growth above by soldering, a seamless copper alloypipe made of phosphorus deoxidized copper with adding Fe is known. Assuch a conventional seamless copper alloy pipe being made of phosphorusdeoxidized copper in which Fe is added as an indispensable component,for example, the following compositions are known. That is, a seamlesscopper alloy pipe for a heat exchanger being made of copper alloycomprising: the total amount of 0.005 to 0.8 weight % of Fe, 0.01 to0.026% of P, 0.005 to 0.3% of Zr, 3 to 30 ppm of oxygen and remainder Cu(refer to Japanese Patent Laid-Open Nos. 39900/1983) and one comprising:0.01 to 1.0 weight % of Fe, 0.005 to 0.6% of at least of one elementselected from Cr, Si, Mn, As, Ni and Co, 0.005 to 0.6% of at least ofone element selected from P, Ca and Mg, 0.004 to 0.04% of oxygen andremainder Cu (refer to Japanese Patent Laid-Open Nos. 156719/1977).

These seamless copper alloy pipes are assembled as heat transfer pipesof a heat exchanger and are filled up with a heating medium. The heatexchanger is operated by loading and opening wide condensation pressureto a heating medium. HCFC-type fluorocarbon has formerly been used as aheating medium above but HFC-type fluorocarbon is recently become to usesince HCFC-type fluorocarbon contributes braking an ozone layer of earthand there are no fear about HFC-type fluorocarbon.

SUMMARY OF THE INVENTION

However, the condensation pressure at the time of using HFC-typefluorocarbon as a heating medium needs to be made larger than that ofusing the conventional HCFC-type fluorocarbon as a heating medium. Forexample, when R-22 which is typical one in HCFC-type fluorocarbon isused as a heating medium for a heat exchanger, it is enough that thecondensation pressure of HCFC-type fluorocarbon in heat transfer tube is20 kgf/cm². However, when R-410a which is typical one in HFC-typefluorocarbon is used as a heating medium, the condensation pressure inheat transfer pipe needs 31 kgf/cm² and this value is 1.5 times or morefrom the former value. Under the environment in which such highcondensation pressure was loaded periodically, there were problems thatcracks arose and broke the heat transfer pipe to become the possiblecause of a trouble in long time use since 0.2% proof strength andfatigue strength of former heat transfer pipe were not enough, and thatthe characteristics of heat exchanger fell since the size of the heattransfer pipe largely changed because of shortage of 0.2% proofstrength.

Means for Solving the Problems

In viewpoint of the above, the present inventors proceeded the researchfor the development of a seamless copper alloy pipe for a heat exchangerwhich is consisted by a copper alloy having excellent 0.2% proofstrength and fatigue strength and the following knowledge was obtained.

(a) When Co was independently added 0.02 to 0.2% to phosphorusdeoxidized copper, 0.2% proof strength and fatigue strength of copperalloy increased extremely, and electrical conductivity also increased.

(b) When carbon was added 1 to 20 ppm to phosphorus deoxidized copperwith the addition of 0.02 to 0.2% of Co, 0.2% proof strength and fatiguestrength of copper alloy furthermore increased.

(c) The content of P is preferably 0.01 to 0.5% and furthermore, anoxygen content as an unavoidable impurity is preferably regulated to 50ppm or less.

The present invention was achieved based on the results set forth aboveand characterized as follow.

(1) A seamless pipe having high 0.2% proof strength and fatigue strengthfor a heat exchanger, the seamless pipe being made of copper alloycomprising: a total amount of 0.02 to 0.2 weight % of Co, 0.01 to 0.05%of P, remainder Cu, and unavoidable impurities and, as said impurities,the total oxygen content in said alloy is regulated to 50 ppm or less.

(2) A seamless pipe having high 0.2% proof strength and fatigue strengthfor a heat exchanger, the seamless pipe being made of copper alloycomprising: a total amount of 0.02 to 0.2 weight % of Co, 0.01 to 0.05%of P, 1 to 20 ppm of C, remainder Cu, and unavoidable impurities and, assaid impurities, the total oxygen content of said alloy is regulated to50 ppm or less.

In order to manufacture this seamless copper alloy pipe for a heatexchanger of the present invention, at first, usual electrolytic copperis melted under reducing atmosphere to make a molten low oxygen copperand next, Co and a mother alloy of Co and P are added to said moltencopper to make a molten copper alloy. Furthermore, after addingpredetermined amount of carbon as a mother alloy of Co and C to themolten copper alloy above if needed, said molten copper alloy is castedto make a columnar ingot.

This columnar ingot above is heated to a predetermined temperaturewithin the range from 850° C. to 1050° C. and is formed by a extrusioninto water. Furthermore, cold working and annealing are done to make aseamless copper alloy pipe for a heat exchanger having a predeterminedcross-sectional size.

There is described next the reason why the composition of the copperalloy for a seamless pipe of a heat exchanger according to the inventionis defined as above.

(a) Co

Co is dissolved into the matrix of phosphorus deoxidized copper or formsphosphorous compound phases and is an effective component which enhances0.2% proof strength and fatigue strength of copper alloy above. When theCo content is over 0.2%, electrical conductivity of copper alloy abovebecomes less than 70% IACS and thermal conductivity falls. Whereas, whenthe Co content is less than 0.01%, a desired effect is not obtained.Therefore, the Co content is determined to 0.02% to 0.2% and preferably0.04% to 0.1%.

(b)P

P has the work which makes crystal grain finer by coexisting with Co andtherefore, enhances 0.2% proof strength and fatigue strength. When the Pcontent is over 0.05%, electrical conductivity of copper alloy abovedecreases remarkably. Whereas, when the P content is less than 0.01%, adesired effect is not obtained. Therefore, the P content is determinedto 0.01% to 0.05% and preferably 0.015% to 0.04%.

(c) Oxygen

Oxygen is contained as an unavoidable impurity. When the oxygen contentis over 50 ppm, a coarse oxide is formed in copper alloy above and, as aresult, 0.2% proof strength and fatigue strength are decreased.Therefore, the oxygen content in a seamless copper alloy pipe for a heatexchanger is determined to be 50 ppm or less and preferably 10 ppm orless.

(d)C

C is added to the copper alloy above to arise 0.2% proof strength andfatigue strength much more, if needed. When C is added over 20 ppm, itbecomes difficult to melt and cast the copper alloy above inconventional methods. Whereas, when the C content is less than 1 ppm, adesired effect is not obtained. Therefore, the C content is determinedto 1 ppm to 20 ppm and preferably 1 ppm to 5 ppm.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

After preparing an electrolytic copper as a raw material, said copperwas melted under reducing atmosphere to make a molten low oxygen copperin which the oxygen content was 50 ppm or less and Co and a mother alloyof copper and 15% P were added to said molten copper. Furthermore, apredetermined quantity of a mother alloy of Co and 1% C was added ifneeded and, as a result, a molten copper alloy was prepared. The moltencopper alloy above was casted into the mold to make columnar ingotshaving a dimension with a diameter of 320 mm and a length of 710 mm andan element composition shown in Table I to Table 3.

After this columnar copper alloy ingots above were heated under acondition to keep at 950° C. for 1 hour by a billet heater, solutiontreatment and making original pipes having a dimension with a diameterof 100 mm and a thickness of 10 mm were simultaneously performed byextrusion into a water.

Furthermore, a cold working was carried out to such original pipes aboveto make seamless pipes having a dimension with an inner diameter of 6.5mm and a thickness of 0.25 mm. The obtained seamless copper alloy pipesabove were annealed at 550° C. for 1 hour in a bright annealing furnaceand, as a result, the seamless copper alloy pipes for a heat exchanger(hereinafter referred to as present invention pipes) of No. 1 to No. 14and comparative seamless copper alloy pipes (hereinafter referred to ascomparative pipes) of No. 1 to No. 5 were manufactured. Furthermore,conventional seamless copper alloy pipes in which elemental compositionis shown in Table 3 and Fe was contained as an indispensable component(hereinafter referred to as conventional pipes) of No. 1 to No. 3 wereprepared.

The fatigue strength of these present invention pipes of No. 1 to No.14, comparative pipes of No. 1 to No. 5 and conventional pipes of No. 1to No. 3 were measured by monitoring of the existence of a crackinitiation on said pipes when periodical internal pressure, i.e. 60kgf/cm², was loaded and opened wide of 2×10⁷ times from one opened endsof said pipes of which the other ends were closed. These results areshown in Table 1 to Table 3 and evaluated.

Furthermore, 0.2% proof strength and elongation were measured by atensile test being a method according to JIS Z 2241 in which the tensilespecimens having the same composition of the present invention pipes ofNo. 1 to No. 14, comparative pipes of No. 1 to No. 5 and conventionalpipes of No. 1 to No. 3 were used. These results are shown in Table 1 toTable 3. Furthermore, the electrical conductivity of these copper alloyswere measured by a four probe method being a method according to JIS C3001 using 1 m of measuring length. These results are also shown inTable 1 to Table 3 and heat-conducting characteristics were evaluated.

Advantages

From the results of Table 1 to Table 3, all of the present inventionpipes of No. 1 to No. 14 have not a crack initiation under a periodicalinternal pressure of 2×10⁷ times. However, all of the conventional pipesof No. 1 to No. 3 have a crack initiation under a periodical internalpressure of 1×10⁶ times or less. These results indicate that the presentinvention pipes of No. 1 to No. 14 are excellent in fatigue strength ascompared with the conventional pipes of No. 1 to No.3. Moreover,elongation of the present invent-ion of No. 1 to No. 14 does not have amarked difference as compared with conventional pipes of No. 1 to No. 3.However, all of the present invention pipes of No. 1 to No. 14 areexcellent in 0.2% proof strength as compared with the conventional pipesof No. 1 to No. 3 and moreover, it can be understand that the electricalconductivity of the present invention pipes increased.

However, the comparative pipes of No. 1 to No. 5 which have acomposition separating from the claim of this invention show at leastone undesirable characteristics selected from fatigue strength, 0.2%proof strength, elongation and electrical conductivity as a seamlesscopper alloy pipe for a heat exchanger.

As described above, a seamless copper alloy pipe for a heat exchangeraccording to this invention is effective for a heat transfer pipe sinceit has especially excellent fatigue strength and 0.2% proof strength.Especially, the copper alloy pipes in the present invention can highlycontribute to the spread of the heat exchanger which uses HFC-typefluorocarbon as a heating medium.

TABLE 1 Element Composition (weight %) Existence of a crack Remainder:Cu and unavoidable impurities initiation when 0.2% proof Electrical C Ointernal pressure is strength Elongation Conductivity No. Co P (ppm)(ppm) Fe loaded periodically (kgf/mm²) (%) % LACS Present Invention Pipe1 0.05 0.03 — 30 — Nothing 18.3 43.8 86.4 2 0.08 0.03 — 30 — Nothing19.1 42.6 85.3 3 0.10 0.03 — 30 — Nothing 19.2 42.3 85.6 4 0.14 0.02 —30 — Nothing 19.5 39.8 85.1 5 0.19 0.04 — 30 — Nothing 19.8 39.1 86.2 60.11 0.05 — 30 — Nothing 19.1 40.3 85.8 7 0.02 0.02 — 30 — Nothing 18.146.1 89.2

TABLE 2 Element Composition (weight %) Existence of a crack Remainder:Cu and unavoidable impurities initiation when 0.2% proof Electrical C Ointernal pressure is strength Elongation Conductivity No. Co P (ppm)(ppm) Fe loaded periodically (kgf/mm²) (%) % LACS Present Invention Pipe 8 0.16 0.04  5 30 — Nothing 22.4 40.3 85.3  9 0.07 0.03 10 30 — Nothing22.3 43.1 85.8 10 0.09 0.03  4 30 — Nothing 21.1 42.1 86.3 11 0.14 0.02 2 30 — Nothing 21.4 40.3 85.2 12 0.20 0.04  1 30 — Nothing 20.2 41.186.1 13 0.12 0.04 19 30 — Nothing 22.5 41.3 85.2 14 0.03 0.02 15 30 —Nothing 20.1 45.2 88.5

TABLE 3 Element Composition (weight %) Existence of a crack Remainder:Cu and unavoidable impurities initiation when 0.2% proof Electrical C Ointernal pressure is strength Elongation Conductivity No. Co P (ppm)(ppm) Fe loaded periodically (kgf/mm²) (%) % LACS Comparative Pipe 1*0.007 0.04 — 80 — Crack arose at  9.1 41.6 80.5 1 × 10⁵ times 2 *0.70 0.03 — 30 — Nothing 20.3 34.1 65.6 3 0.10 0.03 — *30  — Crack arose at14.7 38.2 86.2 1 × 10⁶ times 4 0.14 *0.005 — 30 — Crack arose at 12.142.6 72.3 2 × 10⁵ times 5 0.09 *0.06  — 30 — Nothing 18.2 36.3 67.2Conventional Pipe 1 0.1  0.03 — 30 *0.1 Crack arose at 13.8 38.4 74.8 2× 10⁶ times 2 *— 0.03 — 30 *0.1 Crack arose at  9.8 39.0 78.2 4 × 10⁵times 3 *— 0.03 — 30 — Crack arose at  6.7 42.3 82.4 1 × 10⁵ times

What is claimed is:
 1. A seamless pipe having high 0.2% proof strengthand fatigue strength for a heat exchanger, the seamless pipe being madeof copper alloy consisting of: a total amount of 0.02 to 0.2 weight % ofCo, 0.01 to 0.05% of P, remainder Cu, and unavoidable impurities and, assaid impurities, the total oxygen content in said alloy is regulated to50 ppm or less.
 2. A seamless pipe having high 0.2% proof strength andfatigue strength for a heat exchanger, the seamless pipe being made ofcopper alloy consisting of: a total amount of 0.02 to 0.2 weight % ofCo, 0.01 to 0.05% of P, 1 to 20 ppm of C, remainder Cu, and unavoidableimpurities and, as said impurities, the total oxygen content in saidalloy is regulated to 50 ppm or less.
 3. The seamless pipe of claim 1,wherein the amount of Co is 0.04% to 0.1% by weight.
 4. The seamlesspipe of claim 1, wherein the amount of P is 0.015% to 0.04% by weight.5. The seamless pipe of claim 1, wherein the total oxygen content is 10ppm or less.
 6. The seamless pipe of claim 2, wherein the amount of C is1 to 5 ppm.
 7. The seamless pipe of claim 3, wherein the amount of P is0.015% to 0.04% by weight.
 8. The seamless pipe of claim 3, wherein thetotal oxygen content is 10 ppm or less.
 9. The seamless pipe of claim 4,wherein the total oxygen content is 10 ppm or less.
 10. The seamlesspipe of claim 7, wherein the total oxygen content is 10 ppm or less.